1. Field of the Invention
The present invention relates to a flow sensor for detecting the flow rate of a fluid in a flow passage.
2. Description of the Related Art
Heretofore, in a Karman vortex type flow sensor, a vortex generator and a sensor element are disposed in series along a flow direction of a fluid in a flow passage, such that when a fluid flows through the interior of the flow passage and Karman vortexes are generated therein, the sensor element detects a flow rate of the fluid based on a frequency (generation period) at which Karman vortexes are generated.
In such a Karman vortex type flow sensor, in order to detect minute flow rates (e.g., flow rates close to zero), it is necessary for the cross sectional area of the flow passage to be designed at a small value for thereby enhancing the Reynolds number (Re) indicated by the following equation (1).Re=(Fluid Inertial Force)/(Fluid Viscous Force)  (1)
In equation (1), the “Fluid Inertial Force” is defined as a force that acts on the fluid separately from the surrounding fluid, whereas the “Fluid Viscous Force” is defined as a force that acts on the fluid in the same manner as the surrounding fluid.
A cross sectional configuration of a conventional flow passage will be described below with reference to FIGS. 21A through 21C.
FIG. 21A is a view showing a case in which a vortex generator 16 is disposed in a flow passage 14 that is circular in cross section. In this case, a columnar vortex generator 16, which is disposed at a central position in the widthwise direction (X direction) of the flow passage 14, is erected in an upstanding manner along the Y direction. Further, ends 16a, 16b of the vortex generator 16 are in contact with a wall portion 14e that forms a wall surface of the flow passage 14.
In this case, a distance in the X1 direction from the side 16c of the vortex generator 16 to the wall portion 14e, and a distance in the X2 direction from the side 16d of the vortex generator 16 to the wall portion 14e are equivalent to each other and are defined by Xe.
With the flow passage 14 having the circular cross sectional shape shown in FIG. 21A, if the diameter (flow passage diameter) of the flow passage 14 is designed to be small, the distance Xe becomes small. In particular, in the vicinity of the ends 16a, 16b of the vortex generator 16, since the ends 16a, 16b and the wall portion 14e are in close contact with each other, the distance Xe is remarkably small.
For this reason, with the cross sectional configuration of FIG. 21A, by making the cross sectional area of the flow passage 14 smaller, the fluid viscous force caused by wall surface resistance of the flow passage 14 rises to a significant extent, more so than the enhancement in the fluid inertial force. As a result, the Reynolds number Re cannot be enhanced and it is difficult to detect minute flow rates close to zero.
FIG. 21B illustrates a case in which the flow passage diameter is designed to be larger than the cross sectional configuration of FIG. 21A. In this case, since the distance Xe is widened, the viscous force in a low flow velocity (small flow rate) region is reduced. However, due to the increase in the cross sectional area of the flow passage 14, the fluid inertial force is lowered, and hence the Reynolds number Re cannot be enhanced. As a result, it is quite difficult to detect minute flow rates close to zero.
Thus, it has been contemplated to design the width W of the vortex generator 16 to be shorter along the X direction, thereby widening the distance Xe. However, if designed in this manner, an alternating force (i.e., a force of the vortex that is generated alternately on the downstream side of the vortex generator 16) and/or the strength (structural integrity) of the vortex generator 16 are lowered. As a result, detection sensitivity of the Karman vortex at the sensor element is decreased, so that detection of Karman vortexes becomes difficult, and durability of the vortex generator 16 is degraded.
FIG. 21C illustrates a cross sectional configuration as disclosed in Japanese Laid-Open Patent Publication No. 11-006748.
As shown in FIG. 21C, in order to solve the aforementioned problems, the flow passage 14 is set to have an elliptical shape in cross section. More specifically, the dimension (lateral direction) of the flow passage 14 in the widthwise direction (X direction) of the vortex generator 16 is set to be longer than the dimension (vertical dimension) Yg of the flow passage 14 in the axial direction (Y direction) of the vortex generator 16.
In this case, the flow passage 14 is defined by two wall portions 14g, 14h facing ends 16a, 16b of the vortex generator 16, and two wall portions 14i, 14j facing respective sides 16c, 16d of the vortex generator 16. Each of the wall portions 14g, 14h is formed linearly or in a straight shape along the X direction, whereas each of the wall portions 14i, 14j is formed in a semicircular shape, so as to facilitate smooth flow of the fluid in the flow passage 14.
In accordance with such a cross sectional configuration, in the structure of Japanese Laid-Open Patent
Publication No. 11-006748, even if the cross sectional area of the flow passage 14 is designed to be small, the distance Xg between the sides 16c, 16d of the vortex generator 16 and the wall portions 14i, 14j can be made larger. As a result, together with enhancing the fluid inertial force, the fluid viscous force is reduced, thereby enabling minute flow rates close to zero to be detected.